The present invention relates to incrementing systems particularly of the type used in conjunction with a punch card preparation device. Such an incrementing system is the subject of the co-pending patent application of Albert Romeo et al. entitled Card Advancement Mechanism, bearing Ser. No. 402,328 and filed Oct. 1, 1973, the contents of which are incorporated herein by reference. Incrementing systems in the nature of that disclosed in the aforementioned Romeo et al. appplication are adapted to advance a punch card on a selective rather than an incremental basis. As a result of the selective advancement capability such indexing systems are particularly susceptible to errors such as result from a failure to observe mechanical tolerances in the fabrication or assembly of machine components since the results of such errors are usually reflected in a cumulative manner. Such errors may also be caused by variations in humidity conditions.
A punch card constitutes a convenient medium for storing information both at the input and output of a data processing system. Punch cards come in a variety of lengths, the most common of which is the 80 column version so designated because of a corresponding number of columns which are evenly spaced over the length of the card; each column comprises 12 rows such that the punch card in total yields some 960 possible punch positions.
Various punching techniques are available, including gang punching, whereby information may be entered into any or all of the 960 punch positions simultaneously. Serial punching techniques are also available whereby all positions of a single row or a single column may be simultaneously punched. Variations of the gang and serial punching techniques are also known. In the preferred embodiment of the present invention information is simultaneously punched into the punch positions comprising two cards columns.
Because of the large amount of information generated within a data processing system there is a need to expedite the preparation of punch cards; however, this latter need is secondary to the need to accurately position the punch information relative to the face of the punch card. Mechanical tolerances of the punching mechanism are extremely small in order to insure that information being entered into the punch card will be correctly interpreted in a subsequent processing operation. Thus, the punch aperture separation between adjacent columns generally employed is 0.087 inch, measured from center to center of adjacent punch apertures. A tolerance of .+-. 0.010 inch is permitted; this tolerance being dictated in part by the resolution power of the punch card reading equipment. The latter may register an error condition if the differences in punch aperture spacing exceeds the allowed tolerance. It is therefore important that the center line of the aperture be accurately determined. For this purpose the center line of each aperture may be located relative to the leading edge of the punch card by the card reading mechanism. It is possible that an error introduced between adjacent punch apertures will be compounded over the length of the punch card, thus resulting in an error condition being detected in the subsequent processing of the punch card. This is particularly true of systems wherein the punch card is selectively indexed through the punch or read stations. In selective indexing a punch card is stopped at only those columnar positions at which information is to be entered.
In the selective indexing systems once a punch card has been positioned at the punch station and the leading edge or first columnar position has been brought into registration with respect to the indexing system the further advancement of the punch card proceeds somewhat independent of any reference indicia or reference point appearing thereon. Thus, a punch card is translated from one columnar position to another by rotating an index wheel through a predetermined amount such that the periphery of the index wheel, which is in contact with the punch card, advances the latter to the next punch location whereat information is to be entered. After entry of the information therein, the punch card is again rapidly advanced to the next columnar position at which information is to be entered. This may mean the punch card is incremented one columnar position between successive stops (in the preferred embodiment of the present invention the minimum increment would be two columnar positions) or it may be advanced any number of columnar positions within the limit of the balance of columnar positions remaining on the card.
The indexing system is theoretically designed to advance a punch card in exact columnar increments; however, various influencing factors are known to result in the development of a displacement, or error factor, between the actual advancement and the intended advancement. The mechanical tolerance related to the diameter of the index wheel is one factor which contributes to the development of displacements in such a selective indexing system; however, also critical is the possible affect of high humidity on the card length.